Switch to sphinx-better-theme

Includes #721 to avoid conflicts and so I could develop with livereload. It could easily be rebased back onto master.

# ![Demo](https://dl.dropboxusercontent.com/u/360865/mrjob_docs/index.html)

While doing the conversion and painstakingly checking every page, I found some ways I can improve `sphinx-better-theme`. I decided to just add them to mrjob's custom stylesheet for now rather than do a release that I'd have to go back and change if we found more improvements during this review.

BEFORE MERGING: figure out the situation with making `sphinx-better-theme` available to readthedocs.org. Might need to use a submodule or something.